Natural clays, including kaolin clays, frequently include discoloring contaminants in the form of iron and/or titanium-based impurities. Quantities of titanium-based impurities are particularly significant in the case of the sedimentary kaolins of Georgia, where such impurities are commonly present as iron-stained aratase and rutile. In order, therefore, to refine the clay and bring the brightness characteristics of the resultant product to a level acceptable for such applications as paper coating, various techniques have been used in the past to remove such discoloring impurities. Thus, for example, hydrosulfites have been widely used for converting at least part of the iron-based (or "ferruginous") impurities to soluble forms, which may then be removed from the clay.
Among the most effective methods for removing titaniferous impurities, including, e.g., iron-stained anatase, are the well-known froth flotation techniques. Flotation is, of course, a technique commonly used in the ore-separation art for the treatment of ores, and a wide variety of so-called collecting agents have been proposed for ore flotation, such as the di-thiocarbonates and related compounds disclosed in Harris U.S. Pat. No. 3,590,996, and Harris et al, U.S. Pat. No. 3,853,751. When flotation methods are applied in the kaolin art, an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value, for example, by addition of ammonium hydroxide, and a collector or collecting agent is added. The slurry is then conditioned by agitating the same for a sustained period. A frothing agent, for example, pine oil, is added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell, to effect separation of the impurities along with the collector agent. After the flotation step, the flotation beneficiated clay may be subjected to an additional treatment or treatments to remove or modify further discolorants--hydrosulfite bleaching. Similarly, in Nott, U.S. Pat. No. 3,974,067, the purified product from the flotation treatment is subjected to magnetic separation, to remove further magnetically responsive discolorants.
Details regarding the use of flotation techniques for removing titanium-based impurities from kaolins, may be found at numerous points in the prior art, including, for example, in U.S. Pat. No. 3,450,257 to E. K. Cundy. The Cundy disclosure is illustrative of goodly portions of the prior art, wherein fatty acids such as oleic acid, have been utilized as the collector agents appropriate and effective for use where titanium-based impurities are to be removed from kaolin clays. Further details of flotation treatment are set forth in numerous other patents of the prior art, as for example, in U.S. Pat. Nos. 2,990,958; 3,138,550 and 3,450,257.
In kaolin clay products, especially those to be used for paper coating and the like, brightness is indeed an extremely important consideration, and much effort is exerted in improving the brightness of the clay--as measured by the standard G.E. scale (see TAPPI Procedure T-646-os-75). Even an improvement of less than a full point can be very significant.